Here are a cluster of 3-4 km high iceberg mountains called Norgay Montes.
They must be frozen water ice as nitrogen, methane and carbon monoxide can't develop into strong enough structures to support their weight and retain steep angular ridge lines as seen here.
Frozen ice develops a strong crystalline structure that could stand 3-4 km high at Pluto's gravity. Pluto's gravity is far less than earths.
Acceleration of gravity on Earth . . 9.807 m/s2
Click this Youtube video by Mattias Malmer to see a beautiful 360 degree rendering of this theoretical "volcano" on Pluto called Wrights Montes.
This land is soft as there is only one impact crater meaning this surface has been recently renewed.
The icebergs on the right appear to be colliding with this softened land, in turn pushing it up into a mound we call Wright Montes.